The invention relates generally to synthetic jets and, more particularly, to a method and apparatus of acoustic noise reduction therein.
A synthetic jet may influence the flow over a surface to control flow, as in, for example, separation from an airfoil, or to enhance convection on a surface. A typical synthetic jet actuator includes a housing defining an internal chamber, and an orifice is present in a wall of the housing. The actuator further includes a mechanism in or about the housing for periodically changing the volume within the internal chamber so that a series of fluid vortices are generated and projected in an external environment out from the orifice of the housing. Various volume changing mechanisms include, for example, a piston positioned in the jet housing to move so that gas or fluid is moved in and out of the orifice during reciprocation of the piston and a flexible diaphragm as a wall of the housing. The flexible diaphragm is typically actuated by a piezoelectric actuator or other appropriate means.
Typically, a control system is utilized to create time-harmonic motion of the diaphragm. As the diaphragm moves into the chamber, decreasing the chamber volume, fluid is ejected from the chamber through the orifice. As the fluid passes through the orifice, the flow separates at the sharp edges of the orifice and creates vortex sheets which roll up into vortices. These vortices move away from the edges of the orifice under their own self-induced velocity. As the diaphragm moves outward with respect to the chamber, increasing the chamber volume, ambient fluid is drawn from large distances from the orifice into the chamber. Because the exiting vortices get convected away from the edges of the orifice, they are not affected by the ambient fluid being entrained into the chamber. Thus, as the vortices travel away from the orifice, they synthesize a jet of fluid, thus called a “synthetic jet,” through entrainment of the ambient fluid.
A synthetic jet may be used for thermal management of tight spaces where electronics may be housed and where space for the electronics is a premium. Typically, wireless communication devices such as cellular phones, pagers, two-way radios, and the like, have much of their heat generated in integrated circuit (i.e. IC) packages that are positioned in such tight spaces. Because of the limited space and limited natural convection therein, the heat generated is typically conducted into printed circuit boards and then transferred to the housing interior walls via conduction, convection, and radiative processes. The heat is then typically conducted through the housing walls and to the surrounding ambient environment. The process is typically limited because of the limited opportunity for convection cooling within the housing and over the printed circuit boards. The low thermal conductivity of the fiberglass epoxy resin-based printed circuit boards can lead to high thermal resistance between the heat source and the ambient environment. And, with the advent of smaller enclosures, higher digital clock speeds, greater numbers of power-emitting devices, higher power-density components, and increased expectations for reliability, thermal management issues present an increasing challenge in microelectronics applications.
To improve the heat transfer path, micro/meso scale devices such as synthetic jets have been proposed as a possible replacement for or augmentation of natural convection in microelectronics devices. Applications may include impingement of a fluid in and around the electronics and printed circuit boards. However, a synthetic jet typically has two natural frequencies at which the synthetic jet yields superior cooling performance. These natural frequencies include the structural resonant frequency and the acoustic resonance (Helmholtz) frequency. The structural resonant frequency is caused at the natural frequency of the structure of the synthetic jet, which consists typically of the synthetic jet plates acting as a mass and the elastomeric wall acting as a spring. The acoustic resonance frequency is characterized by the acoustic resonance of air mass flowing in and out of the synthetic jet orifice. The effect is due to the air in the synthetic jet volume acting as a spring and the air in the orifice acting as a mass. The acoustic resonance is expectedly accompanied by a loud tonal noise and a determined vibrational mode if the two modes are not separated from one another in the frequency domain. Thus, the process of operating a synthetic jet typically results in a loud noise that may limit or preclude its use in cooling and other applications.
Therefore, it would be desirable to design an apparatus and method for reducing acoustic noise in a synthetic jet while not compromising performance thereof.